Fouling is generally defined as the accumulation of unwanted materials on the surfaces of processing equipment. In petroleum processing, fouling is the accumulation of unwanted hydrocarbon-based deposits on heat exchanger surfaces. It has been recognized as a nearly universal problem in design and operation of refining and petrochemical processing systems, and affects the operation of equipment in two ways. First, the fouling layer has a low thermal conductivity. This increases the resistance to heat transfer and reduces the effectiveness of the heat exchangers. Second, as deposition occurs, the cross-sectional area is reduced, which causes an increase in pressure drop across the apparatus and creates inefficient pressure and flow in the heat exchanger.
Fouling in heat exchangers associated with petroleum type streams can result from a number of mechanisms including chemical reactions, corrosion, deposit of insoluble materials, and deposit of materials made insoluble by the temperature difference between the fluid and heat exchange wall. For example, the inventors have shown that a low-sulfur, low asphaltene (LSLA) crude oil and a high-sulfur, high asphaltene (HSHA) crude blend are subject to a significant increase in fouling when in the presence of iron oxide (rust) particulates.
One of the more common root causes of rapid fouling, in particular, is the formation of coke that occurs when crude oil asphaltenes are overexposed to heater tube surface temperatures. The liquids on the other side of the exchanger are much hotter than the whole crude oils and result in relatively high surface or skin temperatures. The asphaltenes can precipitate from the oil and adhere to these hot surfaces. Another common cause of rapid fouling is attributed to the presence of salts and particulates. Salts/particulates can precipitate from the crude oils and adhere to the hot surfaces of the heat exchanger. Inorganic contaminants play both an initiating and promoting role in the fouling of whole crude oils and blends. Iron oxide, iron sulfide, calcium carbonate, silica, sodium and calcium chlorides have all been found to be attached directly to the surface of fouled heater rods and throughout the coke deposit.
Prolonged exposure to such surface temperatures, especially in the late-train exchanger, allows for the thermal degradation of the organics and asphaltenes to coke. The coke then acts as an insulator and is responsible for heat transfer efficiency losses in the heat exchanger by preventing the surface from heating the oil passing through the unit. Salts, sediment and particulates have been shown to play a major role in the fouling of pre-heat train heat exchangers, furnaces and other downstream units. Desalter units are still the only opportunity refineries have to remove such contaminants and inefficiencies often result from the carryover of such materials with the crude oil feeds.
Blending of oils in refineries is common, but certain blends are incompatible and cause precipitation of asphaltenes that can rapidly foul process equipment. Improper mixing of crude oils can produce asphaltenic sediment that is known to reduce heat transfer efficiency. Although most blends of unprocessed crude oils are not potentially incompatible, once an incompatible blend is obtained, the rapid fouling and coking that results usually requires shutting down the refining process in a short time. To return the refinery to more profitable levels, the fouled heat exchangers need to be cleaned, which typically requires removal from service, as discussed below.
Heat exchanger in-tube fouling costs petroleum refineries hundreds of millions of dollars each year due to lost efficiencies, throughput, and additional energy consumption. With the increased cost of energy, heat exchanger fouling has a greater impact on process profitability. Petroleum refineries and petrochemical plants also suffer high operating costs due to cleaning required as a result of fouling that occurs during thermal processing of whole crude oils, blends and fractions in heat transfer equipment. While many types of refinery equipment are affected by fouling, cost estimates have shown that the majority of profit losses occur due to the fouling of whole crude oils, blends and fractions in pre-heat train exchangers.
Heat exchanger fouling forces refineries to frequently employ costly shutdowns for the cleaning process. Currently, most refineries practice off-line cleaning of heat exchanger tube bundles by bringing the heat exchanger out of service to perform chemical or mechanical cleaning. The cleaning can be based on scheduled time or usage or on actual monitored fouling conditions. Such conditions can be determined by evaluating the loss of heat exchange efficiency. However, off-line cleaning interrupts service. This can be particularly burdensome for small refineries because there will be periods of non-production.
The need exists to be able to prevent the precipitatation/adherance of particulates and asphaltenes from the heated surfaces before the particulates can promote fouling and the asphaltenes become thermally degraded or coked. The coking mechanism requires both temperature and time. The time factor can be greatly reduced by keeping the particulates away from the surface and by keeping the asphaltenes in solution. Such reduction and/or elimination of fouling will lead to increased run lengths (less cleaning), improved performance and energy efficiency while also reducing the need for costly fouling mitigation options.
Some refineries and crude schedulers currently follow blending guidelines to minimize asphaltene precipitation and the resultant fouling of pre-heat train equipment. Such guidelines suggest blending crude oils to achieve a certain relationship between the solubility blending number (SBN) and insolubility number (IN) of the blend. The SBN is a parameter relating to the compatibility of an oil with different proportions of a model solvent mixture, such as toluene/n-heptane. The SBN is related to the IN, which is determined in a similar manner, as described in U.S. Pat. No. 5,871,634, which is incorporated herein by reference. Some blending guidelines suggest a SBN/IN blend ratio >1.3 and a delta (SBN−IN)>10 to minimize asphaltene precipitation and fouling. However, these blends are designed for use as a passive approach to minimizing asphaltene precipitation.
Attempts have been made to improve the method of blending two or more petroleum oils that are potentially incompatible while maintaining compatibility to prevent the fouling and coking of refinery equipment. U.S. Pat. No. 5,871,634 discloses a method of blending that includes determining the insolubility number (IN) for each feedstream and determining the solubility blending number (SBN) for each stream and combining the feedstreams such that the SBN of the mixture is greater than the In of any component of the mix. In another method, U.S. Pat. No. 5,997,723 uses a blending method in which petroleum oils are combined in certain proportions in order to keep the SBN of the mixture higher than 1.4 times the IN of any oil in the mixture.
As described more fully in co-pending U.S. patent application Ser. No. 11/506,901, the inventors have found that feeding a blend of a base crude oil and a predetermined amount of a high solvency dispersive power (HSDP) crude oil, having a total acid number (TAN) of at least 0.3 and a solubility blending number (SBN) of at least 95, will decrease fouling associated with asphaltenes and particulate induced/promoted fouling.
However, of the hundreds of crude oils produced worldwide, only a small number have been identified that meet the current criteria of a HSDP crude oil, which requires a total acid number (TAN) of at least 0.3 and a solubility blending number (SBN) of at least 95. Therefore, there is a continued need for developing alternative approaches to increase the fouling mitigation and online-cleaning effects of non-HSDP or “near-HSDP” crude oils. Such an approach to improvement of non-HSDP or “near-HSDP” would increase the number of options available to refineries for both fouling mitigation and on-line cleaning effects, which would allow energy and maintenance cost savings.